The Devil's Right Hand
by jkwhedon1919
Summary: Contrary to popular belief, Castiel has a plan when he says yes to the devil.


**Disclaimer: I do not own Supernatural.**

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The first thing Castiel does is hide the knowledge of his possession. It's important because if Lucifer makes him forget that, then the game is lost. Lucifer is conversing with Sam, Dean, Crowley and Rowena now. Something about a train wreck. Castiel isn't sure, but as long as the devil is distracted, he's got a fighting chance.

It's painful, hacking his own mind. It may be with his consent this time, but it's an intrusion nonetheless. He has to replicate the information too. It's undesirable, because he will certainly recognize a fake memory later on, but he can't have all the copies be tied to the original data point, not when he expects Lucifer to delete his memory of their fight in Hell. He writes the information in Dothraki so he'll trust its source later on. There's a decent chance no other angel speaks the language, save Metatron, and Metatron isn't an angel anymore.

Some of his hiding spots are obvious- places he expects Lucifer to look. He adds an extra entry in the list of seals the demons broke during the Apocalypse. He encodes a constellation into his memory of Earth's night sky. He tacks it onto the record of possible prophets from 2008. But, he also puts the information in the middle of his first peanut butter and jelly sandwich. He adds it to the address of the whorehouse Dean took him to the night they captured Raphael. He rewrites a conversation with Hannah to include a discussion about Lucifer's possession, and he makes a whole new radio station in the Impala.

He's halfway through adding his knowledge to the relativity equations when Lucifer finds him. The devil has already used his vessel to say goodbye to Sam and Dean, and he's currently walking back to Hell for reasons Castiel does not know.

"I'm taking care of loose ends," Lucifer says, before slashing the equation out of Castiel's hands and grabbing him by the throat.

"Relativity?" he asks. "I guess, that's a good choice; you'd be reminded every time you managed to look outside our vessel. But honestly, it just seems a little incompatible. Divinity interfering with scientific law."

Lucifer walks into Crowley's throne room. Castiel can feel his fingers around Rowena's neck.

"Einstein believed in G-d," Castiel chokes out in his mind. He wonders, vaguely, why he is even feeling the effects of oxygen deprivation right now. His form doesn't need to breathe. There is no oxygen in Heaven.

"You're experiencing this in your memory of Earth," Lucifer explains, looking around at the various trees in Yosemite National Park.

"Pretty much all your memories are of Earth actually. You're very odd, for an angel," he continues. "Anyway, you're experiencing this as if a much more powerful and, if I may say, _handsome_ , angel is choking you in your vessel. I'm simultaneously overpowering your grace while breaking the human you are inhabiting. You loose power, become weak, are unable to fix your vessel, and voila!" He finally lets go of Castiel's neck, allowing him to tumble onto the forest floor. "Asphyxiation."

Castiel gasps for air and stares up at his brother. He has Sam Winchester's face and voice, but that's where the resemblance ends. The devil has cloven hooves, silky, brown fur, inky black scales that somehow reflect blues and purples. He towers over Castiel, examining him with a kind of detached interest.

"Yes, it is impressive, isn't it?" he asks. "And to think, all Sam will ever see is the face of some drunken low-life with suicidal tendencies." He examines Castiel's addition again and sighs.

"It's not even elegant, what you did," he complains. "You just added a comment to the end of the equations. 'E=MC2', and then there's this kind of gibberish, and my name. That's it? There's no poetry! No calligraphy! You could have at least added a line from 'Sympathy for the Devil!'"

Castiel struggles to his feet, but remains silent. The devil's claws slash through Castiel's edits to Einstein's equations and Castiel uses the moment to delete his knowledge of the other changes he's made to his memory.

His head feels as if it has been split open by Naomi's instruments again. He doubles over, so he gets a full view of the devil's hoof as it connects to his face.

"What did you do?!" Lucifer thunders, and Castiel, who is still trying to remember basic grammar, is not too surprised to discover he doesn't have an answer.

"I don't know," he admits hesitantly.

Lucifer throws him yet again, but this time, he falls into a small corner of his vessel that his mind has constructed to look like Heaven's jail. Lucifer hurls the cell door closed and breathes heavily as he looks around the stone hallway.

"This is Heaven?!" he screams. "What is with our father and cages?!" He begins pacing the length of floor in front of Castiel's cell. "His precious chosen hadn't even _invented_ cages when he built this place! They were still hiding in caves and killing each other with rocks." He viciously kicks at a nearby boulder and Castiel is pleased to find that the pain in his head has decreased to a persistent throb.

"I suppose when you view time from a non-linear perspective, you're free to use the inventions of any era at any time," he supplies. "To our father, human beings are always inventing cages, yet, simultaneously, they are also always inventing cell phones."

Lucifer abruptly stops his pacing to observe Castiel. For a moment, Castiel believe the devil will strike again, but instead, he takes a deep steadying breath and narrows his eyes at his captive.

"What did you do?" he asks again.

"I. Don't. Know," Castiel responds defiantly. Lucifer will know that he is not lying. Angel's minds are easy to read, and while he shares a vessel with Lucifer, his thoughts might as well be presented on billboards with sparkling lights.

Lucifer's eyes widen and he even takes a step back, now staring at Castiel with interest and, possibly, respect?

"Clever," he whispers. "What else did you change?" He pauses. "No, I suppose you won't know that either."

Lucifer makes his way through the torture chambers of Hell. One by one, he steals the shredded soul of each and every demon that crosses his path. Those attending Crowley, the ones doing the maiming, the ones on the rack- all are swallowed up into his brother's form.

Castiel blinks and returns to what is happening in his cell. Something had been changed in his mind? But who could do that right now? Even for Lucifer, Castiel's mind is strictly read-only, he could create illusions, yes, but he couldn't actually _delete_ something, unless-

"Yes!" Lucifer groans loudly. He holds two of his hands behind his back and raises his eyes to the ceiling as if in prayer before rounding on Castiel. "You did it to yourself! By father, you're slow!"

Of course! That would make sense. The only way to protect what he knew from Lucifer was to make him _un-know it_. It was crass, and it would hurt terribly, but-

"Yes, yes, you're a martyr," Lucifer says, waving his hand. "And you don't need to worry about the First Blade's location. It's not like _that_ can kill the Darkness." He shakes his head and resumes his pacing. "It wouldn't even work," he says irritably. "There's no human that bears the Mark of Caine."

"What is your plan?" Castiel spits. Lucifer has now consumed a couple hundred demonic souls and is strutting toward Hell's exit.

"Find Dean." The devil opens the door to Earth.

"Use him to catch Auntie Amara." He walks through the garage.

"Kill the Darkness." Lucifer unfurls his wings, and prepares to fly.

"No." Castiel commands.

Lucifer ceases his preparations and turned toward Castiel, once again in Heaven's jail.

"Excuse me?" he seethes. "Have you forgotten you don't get a vote any more?"

He takes a step into the ether. Castiel can't stop him. Can't move his body a millimeter- not from this position in his vessel. But he is still an angel and he still has grace.

So, he does the next best thing. He breaks his left femur.

For half a millisecond, Lucifer is off balance. Then, he heals his leg and flies toward the Winchesters' home.

So, Castiel breaks his spine.

They tumble to the ground, somewhere in Western Nebraska if Castiel's brief glimpse of the highway is anything to go by.

Lucifer tears the door to Castiel's cell off its hinges and punches Castiel in the gut. It hurts. Much more than any of the blows in the cage. Lucifer attacks Castiel's head next. He flings him to the ground and aims another kick to his ribcage.

"Where the _hell_ do you get off?" he growls. "I am an archangel! I can crush you right now!"

"You kill me, there's no one to consent to you using this vessel," Castiel wheezes.

"You don't think I can't find someone else?" the devil asks. "You don't think I can't _convince_ someone else?"

Castiel gives his brother a humorless smile.

"You can, but you won't," he says. "You like this. You _like_ taking over this vessel. You like being able to push me into a little corner of my mind and use my body like a puppet."

Lucifer ceases his blows and Castiel uses the opportunity to sit up slightly. Blood dribbles down his forehead.

"I beat you," he continues. "Maybe it was Sam and Dean who pushed you into the hole, but they wouldn't have done it without me."

Castiel lurches to his feet and uses one hand to support himself against the wall. His head is dizzy and his vision blurs, but his voice is steady.

"I was powerless," he declares. "I was human. All of Heaven was hunting me. You killed me, and I still beat you."

Castiel closes his eyes and leans his head back.

"You won't kill me because it would be too quick. It would mean you wouldn't be able to have your revenge. You wouldn't be able to punish me, to humiliate me, to torture me," he says.

Lucifer steps into Castiel's space. His nose is inches from Castiel's chin. Castiel lowers his head and meets the devil's gaze.

"The easiest way to torture you," Lucifer whispers, "Is to go to that miserable bunker and let you watch as I kill Sam and Dean Winchester using your hands."

"You're not going near Sam and Dean," Castiel declares.

"You can break every bone in this body," Lucifer shrugs. "I can withstand pain." He glances at Castiel's blood-covered cheek and smirks. "Not too mention, you'll soon be too weak to so much as pluck an eyelash."

"All I need to do is withdraw my consent," Castiel breathes. "And you'll be without a vessel."

Lucifer rolls his eyes.

"You're cute, little bro, but like I said, I can always find another body," he replies.

"Do you know of any other bodies God resurrects on such a frequent basis?" Castiel asks.

"He resurrects _you_ ," Lucifer corrected. He shrugs. "Maybe it's because he lost a bet or something. Trust me, it's _not_ because of your fashion sense."

"Every time he resurrects me, he resurrects the vessel," Castile states. He eyes Lucifer carefully. "Go ahead, leave. Seek out some other poor soul." He pauses and examines the cuff of his trench coat.

"But if it were me," he says slowly. "Not quite at full strength. Battling someone as old and powerful as God himself, someone I'm not sure I can even beat?" He raises his head and meets his brother's glare. "I'd want an insurance policy."

Lucifer steps out of the cell and resumes his pacing. He doesn't try to take flight again. He just paces. Back and forth. He traverses the length of Castiel's cell 361 times before stopping just outside the doorway.

"I could always dump you in a fantasy world," he says. "Leave you on a beach somewhere- or actuarial convention, given your attire- and move around as I please."

Castiel snorts.

"You've already tried to do that." He replies. "It's funny. I can't remember saying yes anymore, but I still know you're possessing me." He gives his brother a small smile. "I don't know how I know it. I just think of something utterly unconnected, and… I know. Maybe it's a coincidence, but I think we both know it correlates nicely with that headache I can't remember giving myself."

Lucifer frowns. Images flash through Castiel's mind. Meeting Balthazar, the fight in Stull Cemetery, flying across the planet looking for his father. When his gaze returns to the hallways of Heaven's jail, Lucifer is still standing in front of him, frowning, arms folded across his chest.

"So, you maintain your consent?" he says slowly. "As long as I don't touch your little pets?"

Castiel takes a shaky step forward.

"You don't have any communications with the Winchesters," he corrects. "You don't go near them. You don't leave them messages. You don't enter their dreams. You don't answer my cell phone when it rings." His hand grips the walls of his cell once again as his balance falters.

"If you happen to see them while you're on Earth, _you_ fly away." He feels his mouth form into a hard line as he stares at his brother.

"You leave them alone. You do whatever you want with the rest of the world, but you leave them alone, and you'll have my consent."

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 **A/N: I spent a good two days after the last episode** **trying to figure out why Castiel did what he did. It just didn't make sense for him to agree to be Lucifer's vessel, to let him destroy the world- because that would happen if the devil succeeded- without somehow protecting the Winchesters. This is what I came up with.**


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